Systems employing data storage elements include, for example, video cameras and image displays. Such systems employ an addressing structure that provides data to or retrieves data from the storage elements. One system of this type to which one embodiment of this invention is particularly directed is a general purpose flat panel display having storage or display elements that store light pattern data. Flat panel-based display systems present a desirable alternative to the comparatively heavy, bulky, and high-voltage cathode-ray tube-based systems.
A flat panel display comprises multiple display elements or "pixels" distributed throughout the viewing area of a display surface. In a liquid crystal flat panel display the optical behavior of each pixel is determined by the magnitude of the electrical potential gradient applied across it. It is generally desirable in such a device to be able to set the potential gradient across each pixel independently. Various schemes have been devised for achieving this end. In currently available active matrix liquid crystal arrays there is, generally, a thin film transistor for every pixel. This transistor is typically strobed "on" by a row driver line at which point it will receive a value from a column driver line. This value is stored until the next row driver line strobe. Transparent electrodes on either side of the pixel apply a potential gradient corresponding to the stored value across the pixel, determining its optical behavior.
U.S. Pat. No. 4,896,149 describes the construction and operation of an alternative type of active matrix liquid crystal array, referred to as a plasma addressed liquid crystal ("PALC") display. This technology avoids the cumbersome and restrictive use of a thin film transistor for every pixel. Each pixel of the liquid crystal cell is positioned between a thin dielectric barrier and a conductive surface. On the opposed side of the thin barrier an inert gas is stored that may be selectively switched from a nonionized, nonconductive state to an ionized conductive plasma through the application of a sufficient electrical potential gradient across the gas volume.
When the gas is in a conductive state, it effectively sets the surface of the thin barrier to ground potential. In this state, the electrical potential across the pixel and thin dielectric barrier is equal to whatever voltage appears on the conductive surface. After the voltage across the gas volume is removed, the ionizable gas reverts to a nonconductive state. The potential gradient introduced across the pixel is stored by the natural capacitances of the liquid crystal material and the dielectric barrier. This potential gradient remains constant regardless of the voltage level of the conductive surface because the thin barrier voltage will float at a level below that of the conductive surface by the difference that was introduced while it was grounded.
Viewed on a larger scale, a PALC display includes a set of channels formed in an insulating plate and containing inert gas under a top plate that contacts the tops of the ribs forming the channel and is sealingly connected around the periphery with the insulating plate. Parallel electrodes extend along the length of each channel at opposed sides. During operation, the gas is ionized and thereby rendered a conductive plasma by the introduction of a large potential gradient between opposed electrodes. This operation occurs many times per second while the display is in operation.
Proper operation and manufacturability of a PALC display depends to a large degree on the electrical and physical properties of the electrodes. For example, to avoid differences in electrical potential along the length of the electrodes during the ionization of the gas, it is desirable that the resistance per unit length of the electrodes be no more than 2 ohms per centimeter (5 ohms per inch). To achieve this small value of resistance per unit length with the tiny cross-sectional area that is available for the electrodes, highly conductive metals such as gold, silver, copper, and aluminum are used.
However, such metals are prone to oxidation and life-reducing sputtering problems. To reduce fabrication costs and prevent sputtering and other problems, copper electrodes are typically employed that are covered with one or more coatings that resist oxidation and sputtering. Other coatings may also be applied to limit discharge current, uniformly distribute the discharge current, improve electrode emission, reduced ionization voltages, decrease discharge initiation time, and decrease ionized gas decay time.
The coatings are applied uniformly to the entire electrode or electrodes and, therefore, do not solve manufacturability or operational problems that are localized to a particular portion of an electrode, such as an end, center, or edge. Moreover, with so many coatings, manufacturing costs and fabrication problems are increased and situations may exist in which certain coatings interact with or reduce the effectiveness of other coatings.
What is needed, therefore, are electrode structures that are readily manufacturable, have low resistances per unit of length, and provide solutions to a variety of electrode-related manufacturing and operational problems, many of which may be localized to particular portions of the electrodes.